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Bernoulli's Principle states that as the speed of air increases, its pressure decreases.

When you blow a big breath near (but not on) the opening of the bag, you increase the speed of the air molecules and create an area of low air pressure. The surrounding high-pressure air rushes in to fill in the low-pressure area you created ("winds will blow from high to low"). As a result, a large quantity of air from the atmosphere is drawn into the bag at the same time as you blow into the bag.

If you blow with your mouth right on the opening, the only air going into the bag is the air from your lungs. You need a lot more breaths to fill the bag this way.


  • Describe the characteristics of air.

  • Explain how air pressure works.

  • Discuss how air pressure affects our daily lives.


  • Per Class as a Demo:
    an official wind bag (long plastic bag in the shape of a tube, 8 ft x 10.5 – the demo is impressive due to the size of the bag)

    Per Student:
    an extra-large garbage bag with the open end scrunched to make a smaller opening

Key Questions

  • What was the difference between how the two bags were blown up?
  • Using Bernoulli’s Principle, where does the extra air that goes into the balloon come from when your teacher blew into the bag?
  • How does putting your mouth further from the opening make a difference?

What To Do

  1. If using a windbag, tie a knot at one end. Ask the class how many breaths it will take to blow up the bag.
  2. Choose a volunteer to blow up the bag (depending on the size of the person, it may take anywhere from 10 to 50 breaths of air). Get the class to count the breaths out loud.
  3. Let all of the air out of the bag and tell the class that you can blow up the bag in one breath.
  4. Ask the student volunteer to gently hold the closed end of the bag.
  5. Hold the open end of the bag approximately 25 cm (10 inches) away from your mouth.
  6. Using only one breath, blow as hard as you can into the bag. Remember to stay about 25 cm away from the bag when you blow.
  7. Quickly seal the bag with your hand so that none of the air escapes.
  8. Without explicitly pointing out how you did it differently from your volunteer, ask another student to repeat what you just did.


  • How would firefighters use this principle to force smoke out of a building using high-powered fans? Firefighters use Bernoulli’s Principle to quickly and efficiently force smoke out of a building. Instead of placing the fans up against the doorway or window, a small space is left between the opening and the fan in order to force a greater amount of air into the building. Firefighters call this “Positive Air Flow.”
  • Check out the Dyson Air Multiplier fan. Can Bernoulli’s Principle help explain why it works?